Bungie accepted the Edge Award for Interactive Innovation (for Halo 3) with a machinima presentation. The video contains an obvious teaser at the end which seems to briefly show what could be a full-fledged map editor for Halo 3.  The video depicts the person delivering thank you's, walking through various environments which appear similar to locations in the campaign without being specifically recognizable.  Furthermore, the entire video shows characters from the campaign arranged (usually in idle animations but occasionally interacting with the speaker), possible suggesting mission editing functionality.  Check out the video at the link.